Nickel coated flake pigments and methods for their preparation



United States Patent US. Cl. 117-54 Claims ABSTRACT OF THE DISCLOSURE Nickel coated flake pigments having a lustrous metallic sheen consisting of a flake substrate (mica or glass) plated with nickel and superimposed on which is an adherent layer of an oxide of either nickel or titanium of sufficient thickness to provide interference colors. The nickel plating is applied to a previously treated (with a solution of a tin salt and a palladium salt) surface of the substrate, such plating being brought about by dispersing the pretreated substrate in a buffered solution containing an 0.31% concentration of a nickel salt, such as a chloride, adding a reducing agent (soduim hypophosphite) to the dispersion, and then oxidizing the nickel surface to form an adherent nickel oxide layer. Optionally, deposition of a film of hydrous TiO on the nickel or nickel oxide coated flakes can be brought about by hydrolyzing a dilute solution of a titanium salt in a suspension of the flakes.

BACKGROUND OF THE INVENTION This invention relates to improved nickel-coated flake pigments of great brilliance and to novel methods for their preparation and conversion to pigments exhibiting brilliant interference colors.

Nickel coated flake pigments and methods for their preparation are described in US. 3,053,683. Coating a mica substrate with hydrous Ti0 and through hydrolysis of a titanium salt solution is disclosed in US. 3,087,828. The coating of a substrate with nickel in accordance with prior procedures is brought about through either volatilization of the metal, vapor phase decomposition of the metal compound, or mechanically co-milling the metal with mica. When the coating is effected by chemical means, careful control must be exercised over the reaction to avoid bath decomposition.

It has now been found that improved, stable nickelcoated flake pigments containing a mica or glass substate with surface areas ranging from 1 to 4.5 m. gram can be prepared by pretreating or surface-conditioning the flake substrate prior to application of the nickel coating and therafter rapidly depositing such coating on the substrate while the latter is maintained in suspension in a nickel salt solution of controlled concentration, and further, that an adherent film of nickel oxide or titanium dioxide can be readily superimposed on such coated product to provide a desired interference color.

SUMMARY OF THE INVENTION Brilliant color nickel-coated mica or glass flake pigments containing from 0.20 to about .70, and particularly from .50 to .70 gram per square meter of flake surface,

Patented Oct. 27, 1970 "ice and over which a thin, adherent layer of interference color-inducing nickel oxide or TiO can be superimposed are obtained in this invention. Application of the nickel plating on the substrate is brought about by first pretreating the substrate with a combination of tin and palladium salts to precondition their surfaces for reception of the nickel coating by suspending the pretreated flakes in a buffered solution of a nickel salt in the range of 0.3% to 1% concentration, with very rapid plating of the nickel on the flake substrate being effected by incorporating in the suspension sufficient sodium hypophosphite as a reducing agent, to yield a bright nickel coated flake pigment containing said 0.20 gram to about 0.70 gram of nickel per square meter of surface. Subsequent oxidation of the nickel coated product under controlled conditions and in an oxygen deficient atmosphere provides an adherent layer of nickel oxide exhibiting bright interference colors, as does the coating of the nickel coated substrate with an adherent layer of hydrous titanium dioxide. On heating the TiO coated product at ZOO-350 C. brilliant golden flake pigments are obtained the color thereof being substantially independent of the thickness of the hydrous TiO layer.

DESCRIPTION OF PREFERRED EMBODIMENT In accordance with this invention brilliant, lustrous nickel-coated mica or glass flake pigments with a metallic sheen are prepared in an aqueous system by plating a nickel film onto a pretreated, surface-conditioned flake substrate by dispersing the substrate in a hot, dilute solution of a nickel salt buffered on the slightly acid side and to which a solution of sodium hypophosphite as the reducing agent is added to induce a very rapid plating reaction with complete exhaustion of the plating bath. In pretreating the substrate to precondition its surface for reception and deposition of the nickel coating, a combination of salt treatment is undertaken. Thus, the flakes are first treated with a dilute (3% to 3%) solution of a. suitable soluble tin, salt, such as stannous chloride, stirred for a short period of time in such solution, and are then filtered and recovered without washing. The substrate is then dispersed in a dilute (.0l% to 0.1%) solution of a suitable soluble palladium salt, such as palladium chloride, stirred for a short period of time therein, filtered, washed and recovered.

The resulting pretreated and surface-conditioned flakes are then suspended in a relatively dilute (0.3% to 1% concentration) of a nickel salt, preferably nickel chloride solution at a temperature ranging from 55 C. to C. A mixture of a suitable buffer, such as sodium acetate, to maintain a very slight acidity and sodium hypophosphite as a reducing agent is then incorporated in the suspension, whereby a very rapid (5-30 minutes) plating reaction and deposition of nickel on the substrate surface takes place to yield a bright nickel-coated flake pigment containing from about 0.20 to 0.70 gram of nickel per square meter of flake surface. The surface of the nickelcoated flakes is then oxidized at about 475 C. under controlled conditions in an oxygen-deficient atmosphere i.e. one containing considerably less oxygen than is found in air (e.g. less the 10 volume percent of oxygen) to give an adherent film of nickel oxide thereon exhibiting bright interference colors. When the nickel layer is in the range of about 0.50 to 0.70 gram per square meter of flake 3 surface, the nickel oxide layer can be made sufliciently thick to yield second-order interference colors of great brilliance. Such colors in contrast to first-order interference colors advantageously retain their brilliance and intensity on being dispersed in a vehicle.

Alternatively, the nickel coated flakes can be coated through deposition with an adherent layer of hydrous TiO through slow hydrolysis of a very dilute (less than 0.1% TiO basis) of a titanium salt, such as titanyl sulfate, and by stirring at room temperature (2025 C.) in the presence of the nickel-coated flakes. The resulting products exhibit desired interference colors. Upon heating the dry TiO film at from 200350 C. conversion to a brilliant golden flake pigment takes place, the color of which is substantially independent of the thickness of the hydrous Ti layer.

The flake pigments thus obtained are useful in the preparation of coating compositions based on oxidizing vehicles such as paints, enamels, and finishes. They are especially useful in lithographic varnishes, alkyd resins and the like, or in non-oxidizing vehicles such as various fortified alkyds, the acrylic resins, nitrocellulose and the like. They are also useful in water based compositions, such as resin emulsion paints, paper coating mixtures and the like. They will be found to be especially useful in pigmenting plastic compositions, to which they impart an exceptionally brilliant sparkle.

In contrast to many prior nacreous flake pigments containing mica or glass flakes as the substrate, both the nickel-coated flakes themselves and those with oxide coatings superimposed thereon are advantageously opaque and have excellent, improved hiding power. Consequently, it is not necessary in use to provide a black layer beneath a composition containing the pigment to intensify the ef fect of the interference colors.

To a clearer understanding of the invention the following examples, in which the parts mentioned are by weight are given. These are merely illustrative and are not to be considered as limiting the underlying principles of the invention.

EXAMPLE I This example illustrates the nickel coating of mica as a substrate, after the mica has been pretreated or sensitized to render its surface receptive to nickel deposition.

Initially one hundred parts of mica substrate is added to a solution of 10 parts of stannous chloride and 34 parts of cone. HCl in 1000 parts of water containing 2 parts of a 1% solution of a non-ionic surfactant (such as one of the nonylphenoxypolyethyleneoxyethanol series). The mixture is stirred for 5 minutes and filtered without washing. The filter cake is then dispersed in a solution of 0.16 part of palladium chloride and 1.4 parts of cone. HCl in 600 parts of water, stirred 5 minutes, filtered and washed.

(The foregoing treatment may be used With any suitable flake pigment, such as mica, varying in particle size as from surface areas of 1.0 m. /gram to 4.5 mF/gram, or glass flakes of similar dimensions, or the like.)

Fifty parts (dry basis) of pretreated mica flakes as above (filter cake with a surface area of 1 m. gram) is dispersed in a plating solution prepared from 50 parts of nickel chloride (NiCl -6H O) dissolved in 225 parts of water; 79 parts of sodium acetate and 69 parts of sodium hypophosphite (NaH PO -H O) are dissolved in 400 parts of water.

These two solutions are mixed and diluted With 14,400 parts of water, to which is added 5 parts of non-ionic surfactant mentioned above, and the solution is heated to 55 C. At this point the pretreated flake pigment is added with good agitation. In about 1 minute, the solution darkens rapidly and there is some evolution of gas with some foaming. After additional stirring for about 5 minutes, the slurry is filtered, washed and dried. The product is a bright nickel-coated flake containing about 20% nickel by weight, i.e. 0.25 gram of nickel per square meter of surface. In coating compositions this product gives a brownish gray finish of brilliant sparkle.

EXAMPLE II A solution of 107 parts of nickel chloride in 485 parts of water is mixed with a solution of 73 parts of sodium acetate and 64 parts of NaH PO -H o in 370 parts of water and diluted with 17,000 parts of 'water containing 5 parts of a 1% solution of the non-ionic surfactant employed in Example I. The resulting solution is heated to C. Thereupon 40 parts (dry basis) of pretreated mica flakes prepared as in Example I and derived from mica flakes with a surface area of 1 m. /gram. is added wtih good stirring. The plating reaction begins in about 3 minutes and is allowed to continue for 15 minutes. A solution of 73 parts of sodium acetate and 64 parts of NaH PO -H O in 370 parts of Water is then added over a period of 15 minutes and stirring is continued for 15 minutes. The charge is then filtered, washed and dried. The product consists of 67 parts of bright nickel-coated mica flakes containing a thicker nickel coating than the product of Example I, i.e. contains 40.1% nickel by weight, corresponding to about 0.67 gram nickel per square meter of surface.

EXAMPLE III The process of Example II is repeated except that the nickel coating is applied to 40 parts (dry basis) of glass flakes with a surface area of about 1 m. /gram., pretreated in accordance with the procedure of Example I applied to mica flakes.

The resulting product has a nickel content of about 40% by weight and is similar in properties to the product of Example II except that due to the smoother surface of the glass flakes compared to the mica flakes it exhibits an even higher sparkle.

EXAMPLE IV A solution of 70 parts of NiCl -6H O in 315 parts of water is mixed with a solution of 62 parts of sodium acetate and 54 parts of NaH PO -H O in 314 parts of water and then diluted with 15,000 parts of water containing 5 parts of a 1% solution of a non-ionic surfactant as in Example I. The resulting solution is heated to 7080 C. To this is then added With good agitation 10 parts (dry basis) of mica flakes pretreated in accordance with Example I and consisting of very fine mica flakes which pass through a 400-mesh screen and have a surface area of about 4.5 m. gm. The slurry begins to foam and darken in about 3 minutes. Stirring is continued for 15 minutes and the slurry is then filtered, washed and dried to give nickel-coated mica flakes containing about 47% of nickel by weight corresponding to about 0.20 gram nickel per square meter of surface. The product has a brilliant sparkle and the finer particle size makes it more suitable than the product of Example I as a pigment for high gloss coating compositions.

EXAMPLE V In a two-step procedure for coating fine mica flakes, a solution of 46 parts of NiCl -6H O in 210 parts of water is mixed with a solution of 32 parts of sodium acetate and 28 parts of NaH PO -H O in parts of Water and diluted with 7500 parts of water containing 5 parts of a 1% solution of a non-ionic detergent described in Example I. The resulting solution is heated to 7080 C. Thereupon 10 parts (dry basis) of pretreated mica flakes (as in Example I) having a surface area of 4.5 m. gm. is added with good agitation. The reaction begins in about 15 minutes and is allowed to continue for 15 minutes; then a solution of 32 parts of sodium acetate and 28 parts of NaH PO -H O in 160 parts of water at 70 C. is added gradually over about minutes. After 15 minutes of additional stirring, the slurry is filtered, washed and dried to give about parts of nickel-plated mica containing about nickel, corresponding to 0.22 gram nickel per square meter of surface. The product is similar to that of Example IV in appearance. The degree of sparkle is affected to some extent by the dilution of the reactants, higher concentration tending toward dullness and greater dilution toward increased brilliance.

EXAMPLE VI A solution of 93 parts of NiCl -6H O in 420 parts of water is mixed with a solution of 63 parts of sodium acetate and parts of NaH PO -H O in 370 parts of 'water and diluted with 14,000 parts of water containing 5 parts of a 1% solution of an Example I non-ionic surfactant.

The resulting solution is heated to 80 C. Thereupon 8 parts (dry basis) of pretreated mica (as in Example I) having a surface area of about 4.5 m. gm. is added with agitation. Stirring is continued for 15 minutes after reaction starts. Then a solution of 63 parts of sodium acetate and 55 parts of NaH PO -H O in 370 parts of water is added over a period of .15 minutes and stirring is continued for 15 minutes. The slurry is filtered, washed and dried to give 30 parts of a brilliant nickel-coated flake pigment containing 73.5% nickel by weight, corresponding to about 0.63 gram nickel per square meter of mica surface.

EXAMPLE VII In this example oxidation of nickel-coated flakes obtained in Example I is undertaken.

One gram of nickel-coated mica flakes from said example (20% Ni) is placed in a glass tube (one end open and the other end with a rounded closure) 3.0 cm. in diameter and 16 cm. long (eg a 100 ml. centrifuge tube). The tube containing the nickel-coated flakes is placed in an inclined tube furnace (inclination about 45 from the horizontal), is slowly rotated while heated to about 475 C. and is held at 475 C. for about 10 minutes. The resulting flakes are coated with nickel oxide and, when cooled, exhibit bright gold interference colors in air, which shift toward blue on longer heating. They are, however, first-order interference colors and lose much of their intensity when dispersed in a coating composition vehicle. Such flakes are useful as pigments in certain applications such as paper coating, where at least some of the particles are not completely wet by the vehicle and thus permit pigment-air interfaces.

It is to be noted in this example that there is no replacement of the air in the tube during the oxidation reaction except as may occur by diffusion. Thus, the major part of the oxidation occurs in an oxygen deficient atmosphere. Oxidation by heating in open air, may result in an uncontrollable reaction, coupled with excessive oxidation and destruction of the coating.

EXAMPLE VIII In this example oxidation is effected of mica flakes having a thicker nickel coating as obtained from Example II. This was carried out in the apparatus of Example VII. One gram of the nickel-coated flakes from Example II (40% nickel0.67 grams Ni/m?) is heated at 475 C. for at least 20 minutes. The resulting products are coated with nickel oxide and, on cooling, exhibit second-order interference colors of great brilliance even on dispersion in a coating composition vehicle. The shade of interference color varies from gold through violet to blue to to green with increasing thickness of the nickel oxide film, and this in turn is determined by the length of the heating cycle. Higher temperatures in the range of 475 C. to 600 C. promote increased oxidation.

EXAMPLE IX In this example a continuous oxidation treatment is applied to nickel-coated flakes, employing a glass tube of 1.75 cm. diameter and 36 cm. length mounted with suitable means for rotation in a tube furnace having a heating chamber about 3 cm. in diameter and about 20 cm. long and inclined about 10 to the horizontal. Nickelcoated flakes are fed into the tube at a uniform rate, as from a suitable screw conveyor, while the rate of rotation of the tube and the inclination thereof are adjusted so that the rate of flow of the flakes through the tube is about 7.5 cm. per minute. A vibrator attached to the tube assists in preventing agglomeration of the flakes as they travel through the tube. The furnace temperature may be varied from 500 C. to 700 C. The use of the nickelcoated flakes of Example VI (74% nickel on flakes with a surface area of 4.5 mF/gm.) and a temperature of 500 C., requires six passes through the tube to give a. nickel oxide coating which exhibits a first-order blue interference color. At 600 C., six passes gives a nickel oxide coating which shows a second-order gold. At 650-700 C., of surface is required to give the desired second-order interference colors, such as gold to violet to green with increased in temperature.

The process is applicable to any type of nickel-coated flake pigment, but a thickness of nickel coating corresponding to 0.50 to 0.65 gram of nickel per square meter of surface is required to give the desired second-order interference colors. I

The process may also be used to produce a nickel oxide coating on the nickel-coated glass flakes of Example III and even at 700 C. there is no sintering of the glass in the brief period required for oxidation.

It is noted that, in this process as in that of Example VIII, there is no introduction of air into the tube beyond that entering by diffusion and convection, both of which must be controlled so that the oxidation is conducted in an oxygen deficient atmosphere, for example, below about 10 volume percent of oxygen.

EXAMPLE X In this example a TiO coating is applied to nickelcoated flakes.

Five parts of nickel-coated flakes of Example I (made from mica with 1 m. gm. surface) is added to a solution of 5 parts of titanyl sulfate solution (15.3% TiO in 1500 parts of Water below 17 C. The slurry is stirred for several hours while warming to room temperature (20- 25 C.). The flakes rapidly take on a gold color as hydrous TiO is deposited on their surface. As more hydrous TiO is progressively deposited, the color first disappears, then reappears and gradually changes to dull violet and then to green. Any free hydrous TiO present may be removed at this point by decantation. The charge is then filtered, washed and dried. The dry flakes are generally multi-colored and lose intensity on dispersion in a vehicle. However, on being gently heated (200350 C.) for a few minutes, the flakes become bright gold and retain their color and brilliance even when dispersed in a vehicle. Moreover, the gold color of the heated product is substantially independent of the interference color exhibited prior to heating.

The combination of treatment with a tin salt followed by a palladium salt has been found to be a necessary preliminary to chemical plating of mica and glass flakes with nickel in accordance with the invention. The exact amounts of salts used are not critical and in the processes described above sufficient excesses are employed to insure satisfactory results with flake substrates of varying surface areas. While this pretreatment is conducted in quite dilute solutions, if desired the concentrations may be varied by a factor of 2 or 3 either way.

While nickel chloride use is preferred as the source of the nickel, other water soluble nickel salts, such as nickel sulfate, or acetate can be used on a stoichiometrically equivalent basis.

The use of sodium hypophosphite as the reducing agent is critical in the invention since no other reducing agent provides equivalent results. Sodium acetate use as a buffer to maintain a constant hydrogen ion concentration and control the acidity of the reaction is well known. Such use is not critical in the invention since other buffers such as various alkali metal carbonates, phosphates, borates, etc. can be used and in varying concentration.

Among the most critical factors in the reaction eniployed in this invention are the extreme dilution of the nickel plating solutions used and the balance between the dilution and the surface area of the flakes being treated. The useful concentration of the nickel salt, especially a chloride in the plating solution should not exceed about 1%, and lower concentrations in the order of 0.5% to 0.3% will be found to be preferred. At the higher concentrations, there is a tendency to dullness in coating, whereas higher dilution promotes brilliance.

The reducing agent can be added in one step with very rapid deposition of the nickel coating, but such form of addition is not preferred because the plating reaction seems to terminate prior to complete utilization of the nickel present. However, the one-step process despite its low efliciency of nickel ion utilization, does favor brilliance. Adding the reducing agent stepwise is preferred because it results in more complete utilization of the nickel salt and considerably thicker nickel coatings.

The utilization of a surfactant, though desirable to insure complete dispersion of the flakes, is not essential or critical to the invention.

It will be noted that, in the examples of nickel coating, the time of reaction is relatively short. This is in direct contrast to the prior electroless nickel coating procedures in which very slow reaction rates are resorted to because of the severe tendency of the plating baths to decompose with sludge formation. It is a surprising feature of this invention that, when a flake substrate with substantial surface area is present, the plating reaction is extremely rapid and exhausts the bath in a very short time without attendant formation of undesirable sludge.

Two forms of oxide coating are described as useful in the invention to impart desired interference colors to the nickel coated flake pigment. In the one, the nickel layer is directly oxidized in part to nickel oxide while in the other a layer of hydrous TiO is deposited. In both a thin adherent film is provided which exhibits interference colors in accordance with the well known physical laws of optics relating thickness of a film and its refractive index to its color. The optical principles which explain interference colors are well known and are discussed in many textbooks of physical optics such as Robert W. Wood Physical Optics, 3rd Edition, New York, MacMillian, 1936, page 198 and described in U.S. 3,087,827 which disclosure is incorporated herein by reference.

The method of directly oxidizing to obtain the desired interference colors is quite critical. It is relatively easy to oxidize a nickel surface by heating in an atmosphere containing oxygen. However, the reaction with nickelcoated flakes is difficult to control in the open air, where it may proceed violently to complete destruction of the nickel film. By operating in a suitable container with limited access to air as herein contemplated, an oxygendeficient atmosphere results and the reaction proceeds uniformly to the degree desired, depending on the time and temperature. Such atmosphere should contain, on the average, less than 10 volume percent of oxygen. Obviously, an equivalent control can be obtained by the introduction of a mixture of gases, say 5% oxygen and 95% nitrogen by volume into the reaction zone. 'In effecting such oxidation, temperatures of about 475 C. are required for satisfactory results, with temperatures up to as high as 700' C. being permissible under some circumstances. The higher the temperature, the more rapid the oxidation.

The examples show oxidation both in a batch process in an agitated bed and also in a continuous process where the flakes are conducted at pre-determined rates through a heated zone in the presence of the oxygen-deficient atmosphere. If desired the oxidation can be carried out in a fluidized bed, using a suitable mixture of oxygen with an inert gas as the fluidizing gas. Thus, one can use, a mixture of 5% oxygen and 95% nitrogen by volume. If desired the nitrogen could be replaced by another inert gas, such as argon.

In depositing a layer or film of hydrous TiO on the nickel-coated flakes, this is conveniently done by the hydrolysis of a dilute solution of a titanium salt. However, the acidity of the usual commercial solutions is sufiicient to attack the nickel coating in the common methods of heat hydrolysis. Consequently, use is necessary of very dilute solutions (less than 0.1% titanyl sulfate based on TiO content) and the hydrolysis is carried out slowly at room temperature.

The hydrous TiO coating deposited on the flake will result in variable interference colors, but the products per se do not show the desired brilliance when dispersed in a vehicle. However, gently heating treatment in the range of 200 C. to 350 C. converts the products, regardless of their initial color, to bright gold flakes which retain their brilliance when incorporated in coating composition vehicles. This gold color does not behave like an ordinary interference color.

We claim:

1. A process for producing an improved nickel-coated pigment, comprising dispersing a surface-conditioned flake substrate, pre-treated with a combination of stannous chloride and palladious chloride metal salts, selected from the group consisting of mica and glass particles having a surface area ranging from 1 to 4.5 m. gm. in a dilute nickel-salt solution maintained at a slight acidity, a temperature of at least 55 C. and containing not more than about a 1% by weight concentration of said salt, and rapidly depositing nickel as a coating on the surfaces of said substrate by incorporating sodium hypophosphite in said solution as a reducing agent.

2. The process of claim 1 in which the nickel salt is nickel chloride with the concentration thereof ranging from 0.3 to 1%.

3. The process of claim 1 in which the amount of nickel coating deposited on the substrate ranges from about 0.20 gram to 0.70 gram nickel per square meter of substrate surface.

4. The process of claim 2 in which the nickel chloride solution is maintained at a temperature ranging from 55 C. to C. and contains sodium acetate as a buffer.

5. The process of claim 4 in which deposition is effected of about 0.50 to 0.70 gram nickel per square meter of substrate.

6. The process of claim 1 in which on completion of nickel deposition, partial oxidation thereof under a temperature of at least 450 C. and in an oxygen-deficient atmosphere, containing considerably less oxygen than is found in air, is undertaken to obtain an interference color pigment.

7. The process of claim 6 in which the oxidation is carried out in an atmosphere containing less than 10% oxygen by volume and at a temperature ranging from 450 C.-700 C.

8. A process of claim 1 in which on completion of nickel deposition a coating layer of hydrous TiO is superimposed by depositing a layer of hydrous TiO on the nickel-coated pigment.

9. A flake pigment having a lustrous metallic sheen comprising a pretreated, surface conditioned flake substrate selected from the group consisting of mica and glass, coated with from 0.20 to 0.70 gram Ni per square meter of substrate surface, superimposed on which is an adherent layer of a metal oxide selected from the group consisting of nickel oxide and hydrous titanium dioxide, said layer being of sufficient thickness to provide an interference color pigment.

10. The product of claim 9 in which the fiake substrate is mica with particles having a surface area ranging from 1 to 4.5 m. /gm.

References Cited UNITED STATES PATENTS 2,266,117 12/1941 Crocker et a1. 1486.3 X 2,487,632 11/1949 Bennett l486.3 2,702,253 2/1955 Bergstrom 117-47 2,872,312 2/1959 Eisenberg 117-100 X 10 3,053,683 9/1962 Yolles 117--100 X 3,071,482 1/1963 Miller 117--159 X 3,087,828 4/1963 Linton 117-100 X 3,212,917 10/1965 Tsu et a1. 11747 OTHER REFERENCES Eishlock, Metal Colouring, Robert Draper Ltd., 1962,

10 RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R. 

